This invention relates to a process for treating wastewater with oxygen, wherein oxygen of a higher concentration than atmospheric oxygen is contacted at least intermittently with the water surface and introduced into the wastewater through an agitation of said water surface.
The use of pure oxygen or oxygen-enriched air instead of conventional aeration enables a considerable increase in efficiency of the activated sludge process for wastewater treatment, although the costs involved in oxygen supply are rather high.
Many methods and types of equipment for oxygenation are well known in the art to which our invention relates, such as disclosed in "Korrespondenz Abwasser" magazine, issue 1975, p. 278 to 287. These methods are carried out by either the introduction of fine oxygen bubbles adjacent the bottom of tanks which are open or covered by gas-tight means, or by supplying oxygen to a gas compartment defined by means of a cover above the wastewater wherein the oxygen is introduced into the wastewater by means of stirrers, surface aerators or similar equipment.
The prior method of supplying oxygen to the covered gas compartment usually requires that a positive pressure of 30 to 50 mm (water column) be maintained and the surplus gas be withdrawn from the compartment. Depending upon the volume and nature of the wastewater to be treated and hence on the oxygen demand, this surplus gas may still contain appreciable amounts of unused oxygen. On the other hand, optimum conditions for biological treatment of the wastewater cannot be achieved because the gaseous products, in particular CO.sub.2, resulting from the metabolic process are unable to escape. While prior procedures are known for expelling such CO.sub.2 in a subsequent stripping stage, such procedures require additional equipment and they do not achieve optimum operating conditions. Such procedures also involve a loss of oxygen and in addition thereto the residual CO.sub.2 remaining in the water until being expelled interferes with an optimum biological balance being established. These disadvantages, in particular the loss of oxygen, which can hardly be prevented from escaping with the surplus gas, are particularly felt when volume and pollution of the wastewater deviate greatly from the normal values for which the plant is designed. A marked drop from such normal values will lead to unnecessarily high concentrations of oxygen in the wastewater and in the surplus gas, whereas a major increase beyond such normal values will result in an insufficient degradation of pollutants.